What is the primary purpose of a vapour barrier in insulation?

The primary purpose of a vapour barrier in insulation is to protect against moisture. Moisture can enter building assemblies through various means, such as diffusion and air leaks, and if it accumulates in insulation or structural components, it can lead to significant problems including mold growth, wood rot, and overall degradation of building materials. By providing a barrier that restricts the movement of water vapor, the vapour barrier helps maintain the integrity and longevity of the building structure.

While it is true that moisture management plays a critical role in maintaining thermal performance, the vapour barrier specifically targets moisture rather than directly preventing heat loss, which is often managed through insulation's thermal resistance. Other aspects of construction, such as fire safety and structural support, are addressed through different materials and methods not directly related to the function of a vapour barrier.